Don't provide backends from __all__ in persistence
Don't provide backends from __all__ in persistence

Token persistence __init__ specified 'backends' in __all__ however this
was never actually imported to be exposed. This could cause issues from
apidoc as there was no code to match the expected symbols.

Removing 'backends' rather than importing it as we shouldn't import code
we don't use.

Change-Id: Ic87371ffb2f65ae8e933f091bb05faaa64a342d2

cernops/keystone,jumpstarter-io/keystone,dstanek/keystone,ajayaa/keystone,vivekdhayaal/keystone,dstanek/keystone,himanshu-setia/keystone,ajayaa/keystone,ilay09/keystone,jonnary/keystone,takeshineshiro/keystone,klmitch/keystone,rajalokan/keystone,mahak/keystone,mahak/keystone,dims/keystone,takeshineshiro/keystone,idjaw/keystone,rajalokan/keystone,jumpstarter-io/keystone,cernops/keystone,roopali8/keystone,maestro-hybrid-cloud/keystone,rajalokan/keystone,vivekdhayaal/keystone,vivekdhayaal/keystone,openstack/keystone,roopali8/keystone,maestro-hybrid-cloud/keystone,ilay09/keystone,jonnary/keystone,mahak/keystone,jamielennox/keystone,himanshu-setia/keystone,openstack/keystone,dims/keystone,klmitch/keystone,idjaw/keystone,ilay09/keystone,openstack/keystone,dstanek/keystone,jumpstarter-io/keystone,jamielennox/keystone